Fundamental coupled travelling wave tube having a periodic permanent magnetic focussing structure

ABSTRACT

A fundamental coupled cavity travelling wave tube including between electron gun and collector a stack of apertured plates assembled to provide a series of successive cavities coupled for the fundamental mode of operation and in which there is a magnetic pole piece at each nth cavity (n is an integer greater than unity), constituted by a relatively thick apertured ferromagnetic plate between two thinner ferro-magnetic apertured plates, the thinner plates extending the magnetic path provided by the thicker plate towards the electron beam path.

United States Patent Esterson et al.

1451 May 20, 1975 [54] FUNDAMENTAL COUPLED TRAVELLING 3.324.339 6/l967 Winslow et a]. 3l5/3.5 X

WAVE A 3,443.146 5/l969 Buck 4 1 1 1 t v 1 1 A .1 5/35 3,448,326 6/1969 Arfin et al. 1 1 1 .1 315/36 PERMANENT MAGNETIC FOCUSSING 3,617,802 11/1971 Manoly 315/35 STRUCTURE 3,755,706 8/l973 Scott 315/35 [75] Inventors: Maurice Esterson; Robin Charles $3232 gg glgf g gggg Primary Examiner-Saxfield Chatmon, .Jr.

Attorney, Agent, or Firm-Baldwin, Wight and Brown [73] Assignee: English Electric Valve Company Limited, Chelmsford, Essex, England 57 ABSTRACT [22] Filed: Feb. 15, 1974 A fundamental coupled cavity travelling wave tube in- [2 H App! 442884 eluding between electron gun and collector a stack of apertured plates assembled to provide a series of suc- [30] Foreign Application Priority Data cessive cavities coupled for the fundamental mode of F611 16 1973 United Kingdom H 7682/73 operation and in which there is a magnetic pole piece at each n cavity (11 is an integer greater than unity), 521 US. (:1. 315/15; 315/535; 335/210 constituted by a relatively thick apertured rr [Sl] Int. Cl. H01j 26/34 magnetic Plate between two thinner ferromagnetic [58] Field of Search H 315/534! 535, apertured plates, the thinner plates extending the mag- 210 netic path provided by the thicker plate towards the electron beam path.

[56] References Cited UNITED 7 Claims, 7 Drawing Figures 3,010,047 ll/l96l Bates INS/3.5

. MX MX\% N s,

% i 3 l X X 1 A Y Y MY H X M3 MY nWY EMY Q t I PATENTEnmzoms 31885192 sum 2 OF 4 PATENTEDMAY201975 3.885192 SHEET *4 n? 4 I FUNDAMENTAL COUPLED TRAVELLING WAVE TUBE HAVING A PERIODIC PERMANENT MAGNETIC FOCUSSING STRUCTURE BACKGROUND AND BRIEF SUMMARY OF THE INVENTION This invention relates to travelling wave tubes and more specifically to travelling wave tubes of the kind which will hereinafter be referred to as fundamental coupled cavity travelling wave tubes." The well-known so-called cloverleaf" and centipede" travelling wave tubes are tubes of this kind. The object of the invention is to provide improved high power travelling wave tubes of the kind referred to in which periodic permanent magnet (P.P.M.) focusing is provided by permanent magnet systems of high efficiency and conveniently small bulk and weight.

The use of P.P.M. focusing in travelling wave tubes in which the wave structure coupled to the electron beam is a helix, is well known and the development of improved magnetic materials, notably Samarium Cobalt, has made the use of this type of focusing even more attractive by enabling P.P.M. focusing systems of reduced weight and bulk to be built. However, the power handling limitations of the helix are such as to render the helix type of tube unsuitable for the attainment of high power levels e.g. powers in the range of l KW to l MW (dependent on frequency).

Travelling wave tubes with coupled cavity structures have been developed in order to enable higher power levels than those obtainable with the helix type of tube to be attained. Coupled cavity travelling wave tubes may be regarded, for present purposes, as divided into two classes, namely the space harmonic coupled cavity type and the type to which the present invention relates, namely the fundamental coupled cavity type. As will be explained later herein the former type lends itself admirably to the use of P.P.M. focusing but grave difficulties oppose themselves to the application of P.P.M. focusing to tubes of the latter type. The present invention seeks to overcome these difficulties.

FIG. I, which is provided for purposes of introductory explanation, is a schematic cross-sectional view, taken on the centre line C/L, showing one magnet length of a known P.P.M. focusing system in a coupled cavity travelling wave tube of the space harmonic coupled cavity type. As will be apparent the cavity coupling plates, which are slotted or apertured in any of the customary ways (though, owing to the nature of the drawing in FIG. I, the slots or apertures do not appear in that figure) are employed as magnet poles. Two of these plates, 1 and 2, are represented in FIG. 1. They are made of soft iron and are provided with cylindrical, drift-tube-like pole pieces IA and 2A which are, of course, aligned and surround the electron beam which is projected through them from an electron gun (not shown) at one end of the axis of the tube to an electron collector (also not shown) at the other end of the said axis. The permanent magnetic field is provided by a ring magnet 3 the polarity of which is conventionally indicated by the letters N and S. In this way the magnetic field is brought very close indeed to the electron beam and, because the distance d between the adjacent ends of the adjacent pole pieces 1A and 2A can be made quite small in relation to the magnetic leakage paths, such as MLl and ML2, which exist, satisfactorily high values of axial magnetic field strength for good P.P.M. focusing can be obtained from a magnetic system of acceptably small bulk and weight, especially if the newly developed magnetic materials, such as Samarium Cobalt, are used for the magnets. However, because there is no room, in a fundamental coupled travelling wave tube, for pole pieces such as the pole pieces IA and 2A, a structure such as that represented in FIG. I, does not solve the problems which face the designer seeking to apply P.P.M. focusing to a travelling wave tube of the fundamental coupled cavity type, i.e. one in which the beam is coupled to the cavity structure in the fundamental mode of the structure.

Consider the case of a typical known fundamental coupled cavity travelling wave tube built-up, in the well known way, of a stack of slotted or apertured copper plates all of the same outside diameter. FIG. 2 shows examples of such plates though, as will be well understood, there are other known forms of plate which could be used. Five plates, reference X1, X2 and X3 and Y1 and Y2 are shown in FIG. 2. The plates X1, X2 and X3 are alike and have tapered radially inward projections P as shown. The plates Y1 and Y2. which are thinner than the plates X1, X2 and X3, are also alike and have central beam passing holes H and radial coupling slots S. The X and Y plates alternate in the stack, the plates being aligned so that their outer surfaces lie in a cylindrical surface with the holes H in alternate plates in the stack also aligned. The relative rotational positions of the plates are such that all the Y plates have their radial slots aligned but successive X plates are rotationally staggered or twisted (through 30 in this particular example, in which each X plate has six inward projections) so that, with an order of stacking of X1, Y1, X2, Y2, X3 and so on, the rotational positions of the plates are as represented in FIG. 2, with plate X2 at 30 to the plates XI and X3 and the plates Y1 and Y2 with their radial slots aligned and also in the rotational positions represented in FIG. 2. The stack is formed as a mechanical unit, for example, by brazing, to form with end members (not shown) a vacuum tight structure, one end member carrying an axially positioned electron gun (not shown) and the other carrying an axially positioned electron collector (also not shown). The resulting structure is well known and constitutes one well-known form of clover leaf travelling wave tube. It will readily be seen that, in such a structure, there is no room for drift-tube-like pole pieces such as the pole pieces IA and 2A of FIG. 1. Further, it will be appreciated that it is not practical, even by making the X plates of magnetic material, such as soft iron, to obtain a satisfactory P.P.M. focusing system after the manner of FIG. I because the separating distance, corresponding to the distance d in FIG. 1, between successive X plates would be as long as the leakage field paths corresponding to paths such as MLI and ML2 in FIG. I.

According to this invention a fundamental coupled cavity travelling wave tube comprising a stack of slotted or apertured plates assembled to provide a plurality of successive cavities coupled for the fundamental mode of operation has, at each nth cavity (where n is an integer greater than unity) a magnetic pole piece constituted by a relatively thick apertured plate made of ferromagnetic material and which is between a pair of thinner apertured plates which are also made of ferro-magnetic material and which extend the ferromagnetic path provided by said relatively thick plate to the close neighbourhood of the electron beam path of the tube.

In use ring shaped external magnets are provided in such positions and are of such lengths as to cooperate to provide permanent magnetic fields through the pole pieces.

Preferably the relatively thick ferro-magnetic plates which form parts of the pole pieces are of larger external diameter than the other plates so that their rims project outwardly from what would otherwise be the smooth cylindrical outer surface of the stack, and ring magnets are positioned each in the space between successive projecting rims.

Theoretically the value of it may be 2 but it is prefera bly at least 3.

It is common practice. in known clover leaf and similar fundamental coupled cavity travelling wave tubes, to taper" the tube towards one end. This may be done in various known ways e.g. by linearly varying the cavity height (i.e. its dimension along the tube axis) or by modifying the slot or aperture dimensions of the plates towards one end of the tube. The former expedient produces what is usually called velocity tapering" and the latter produces what is usually called phase tapering." Tapering may be resorted to, in any manner known per se, when carrying out this invention but, as will be apparent. if variation of the cavity height is adopted corresponding variation of the separation of successive pole-pieces should also be adopted, with corresponding variation of ring magnet length. If tapering is required in a tube in accordance with this invention it is, therefore, preferred to effect it by a method which does not involve variation of cavity height, for if cavity height is uniform from one end of the tube to the other, the sep' aration of the pole pieces and the lengths of the magnets will also be uniform and this has obvious advantages from the point of view of practical manufacture.

BRIEF DESCRIPTION OF THE DRAWING Whereas FIG. 1 illustrates P.P.M. focussing in a prior art space harmonic coupled cavity tube and FIG. 2 illustrates a conventional arrangement of plates in a fun damental coupled cavity tube, the invention is further described by way of example with reference to FIGS. 3 and 4 of the accompanying drawings which illustrate one embodiment of this invention, and with reference to FIGS. 5, 6 and 7 of the accompanying drawings which illustrate modifications and extensions thereof.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 3, which is a view of the same nature as that of FIG. 1, the tube comprises a stack of plates which are assembled to provide a succession of cavities coupled for the fundamental mode of operation. The plates which are shown unshaded in FIG. 3 are made of copper: those which are shown crosshatched in the figure are made of ferromagnetic mate rial preferably of soft iron. The plates referenced Y in FIG. 3 are made of copper and each has a central hole and is radially slotted like the plates Y1 and Y2 of FIG. 2. The plates referenced X in FIG. 3 are also of copper and are apertured to provide a plurality of ta pered radially inward projections like the plates X1, X2 and X3 of FIG. 2. In addition the stack includes plates referenced MY which are like the plates Y except that they are made of ferro-magnetic material, such as soft iron. instead of copper. The stack also includes soft iron plates MX which are also of ferro-magnetic material and each of which is between two adjacent iron plates MY as shown. One of these plates MX is shown in face view in FIG. 4. As will be seen each of the plates MX is like the plates X except that it is made of ferromagnetic material and is of larger external diameter so that. when these plates are assembled in the stack the solid rim" of each projects, as shown in FIG. 3, beyond what would otherwise be the smooth cylindrical outer surface of the stack. Permanent ring magnets 3 are fitted externally of the said cylindrical surface between the projecting rims of adjacent plates MX so that these plates act as pole pieces for the magnets. Such pole pieces are provided at every nth cavity in the example illustrated at every fourth cavity and in this way the pole pieces, each consisting of a plate MX in effect extended towards the electron beam by the plates MY between which it is positioned, serve the required purpose of enabling a sufficiently strong magnetic axial field to be produced in the close neighbourhood of the beam, thus providing satisfactory P.P.M. focusing. The P.P.M. focusing system, consisting of the permanent magnets and their associated pole pieces, is compact. economical of space and of relatively light weight, especially if materials such as Samarium Cobalt, are used in the construction.

The plates used in the stack are, of course, assembled in the manner already described and known per se in clover leaf and like travelling wave tubes: that is to say the radially slotted plates are assembled with their central holes aligned and their radial slots aligned, while the other plates, whether of copper or of ferromagnetic material, are assembled so that the apertures in alternate ones of these plates are aligned while the remaining ones of these plates are staggered or twisted rotationally with respect thereto in the example illustrated. by 30.

FIG. 5 shows a modification of the arrangement of FIG. 3. The outer annular portions of the soft iron plates MX which project beyond the adjacent soft iron plates MY are much thinner than the central portions. This permits the permanent ring magnets 3 to be larger than would otherwise be the case, and allows a higher peak magnetic field to be achieved on the axis of the travelling wave tube.

FIG. 6 shows a further modification which can be adopted for applications in which the operating frequency of the travelling wave tube permits the use of soft iron plates MY which are sufficiently thick to carry the required magnetic flux, without assistance from the plates MX. In this case only sufficient overlap between plates MY and MX is provided to ensure good flux linkage. In FIG. 6 each soft iron plate MXS possesses complete circular symmetry and surrounds copper projections X5 which are brazed to it. Six copper projectors X5 are brazed to each plate MX, and the resulting structure is as illustrated in FIG. 7.

The invention is not limited to the use of plates with the particular configurations of slotting and aperturing illustrated. Other forms of configuration of slotting and aperturing, known per se in fundamental coupled cav ity travelling wave tubes practice, may be adopted.

We claim:

I. A coupled cavity travelling wave tube including a stack of alternately slotted and apertured plates assembled to provide a plurality of successive coupled cavities coupled for the fundamental mode of operation; a

magnetic pole piece provided at each nth cavity (where n is an integer greater than 2) which pole piece consists of an apertured plate made of ferromagnetic material and positioned between a pair of thinner slotted plates which also consist of ferromagnetic material and which have regions extending the ferromagnetic path, provided by the first mentioned apertured plate, close to the electron beam path of the tube, each pair of said thinner slotted plates being spaced apart from each other by the thickness of the first mentioned apertured plate to define a cavity between them; a periodic permanent magnetic focussing structure surrounding said stack and consisting of ring shaped magnets which provide permanent magnetic fields through said pole pieces to said electron beam path.

2. A travelling wave tube as claimed in claim 1 wherein each of said ferro-magnetic apertured plates contains a central aperture into which ferro-magnetic projections, which are extensions thereof project towards the central axis of the travelling wave tube.

3. A travelling wave tube as claimed in claim 1 wherein each of said ferro-magnetic apertured plates contains a central aperture into which copper projections are arranged to project towards the central axis of the travelling wave tube.

4. A travelling wave tube as claimed in claim 1 wherein ring shaped external magnets are provided in such positions and are of such lengths as to co-operate to provide permanent magnetic fields through the pole pieces.

5. A travelling wave tube as claimed in claim 1 wherein the ferro-magnetic apertured plates which form parts of the pole pieces are of larger external diameter than the other plates so that their rims project outwardly from what would otherwise be the smooth cylindrical outer surface of the stack. and ring magnets are positioned each in the space between successive projecting rims.

6. A travelling wave tube as claimed in claim 1 wherein the value of n is at least 3,

7. ln a fundamental coupled cavity travelling wave tube, in combination:

a stack of plates comprising a set of apertured plates each having an outer, rim-like portion and a plurality of radially inwardly directed projections whose tips define a minimum diameter of its aperture and a set of slotted plates having central openings which are of much smaller diameter than said minimum diameter of the apertured plates, said apertured and slotted plates being alternated in said stack so that each apertured plate is sandwiched between two of said slotted plates and a cavity is defined between the two slotted plates sandwiching each apertured plate, each slotted plate having slots coupling the cavities with which it is associated and the slots of all of said slotted plates being aligned while the projections of said apertured plates are rotationally staggered sequentially through said stack to provide the fundamental coupled cavity structure; and

a periodic permanent magnet focussing structure integrated with said stack and comprising a magnetic pole structure at every nth cavity, where n is at least 3, and a ring-like permanent magnet associated with each pair of pole structures, each pole structure comprising an apertured plate having its rim-like portion of extended outer diameter and with at least such rim-like portion being of ferromagnetic material and a pair of slotted plates of ferromagnetic material sandwiching such apertured plate therebetween, the apertured plates and slotted plates which do not form pole structures being formed of electrically conductive, nonmagnetic material and having outer diameters lying on a cylindrical surface from which said extended rim-like portions project, each ring-like permanent magnet embracing said cylindrical surface between a pair of pole structures and having opposite pole faces engaging the extended rim-like portions of such pair of pole structures. 

1. A coupled cavity travelling wave tube including a stack of alternately slotted and apertured plates assembled to provide a plurality of successive coupled cavities coupled for the fundamental mode of operation; a magnetic pole piece provided at each nth cavity (where n is an integer greater than 2) which pole piece consists of an apertured plate made of ferromagnetic material and positioned between a pair of thinner slotted plates which also consist of ferromagnetic material and which have regions extending the ferromagnetic path, provided by the first mentioned apertured plate, close to the electron beam path of the tube, each pair of said thinner slotted plates being spaced apart from each other by the thickness of the first mentioned apertured plate to define a caVity between them; a periodic permanent magnetic focussing structure surrounding said stack and consisting of ring shaped magnets which provide permanent magnetic fields through said pole pieces to said electron beam path.
 2. A travelling wave tube as claimed in claim 1 wherein each of said ferro-magnetic apertured plates contains a central aperture into which ferro-magnetic projections, which are extensions thereof project towards the central axis of the travelling wave tube.
 3. A travelling wave tube as claimed in claim 1 wherein each of said ferro-magnetic apertured plates contains a central aperture into which copper projections are arranged to project towards the central axis of the travelling wave tube.
 4. A travelling wave tube as claimed in claim 1 wherein ring shaped external magnets are provided in such positions and are of such lengths as to co-operate to provide permanent magnetic fields through the pole pieces.
 5. A travelling wave tube as claimed in claim 1 wherein the ferro-magnetic apertured plates which form parts of the pole pieces are of larger external diameter than the other plates so that their rims project outwardly from what would otherwise be the smooth cylindrical outer surface of the stack, and ring magnets are positioned each in the space between successive projecting rims.
 6. A travelling wave tube as claimed in claim 1 wherein the value of n is at least
 3. 7. In a fundamental coupled cavity travelling wave tube, in combination: a stack of plates comprising a set of apertured plates each having an outer, rim-like portion and a plurality of radially inwardly directed projections whose tips define a minimum diameter of its aperture and a set of slotted plates having central openings which are of much smaller diameter than said minimum diameter of the apertured plates, said apertured and slotted plates being alternated in said stack so that each apertured plate is sandwiched between two of said slotted plates and a cavity is defined between the two slotted plates sandwiching each apertured plate, each slotted plate having slots coupling the cavities with which it is associated and the slots of all of said slotted plates being aligned while the projections of said apertured plates are rotationally staggered sequentially through said stack to provide the fundamental coupled cavity structure; and a periodic permanent magnet focussing structure integrated with said stack and comprising a magnetic pole structure at every nth cavity, where n is at least 3, and a ring-like permanent magnet associated with each pair of pole structures, each pole structure comprising an apertured plate having its rim-like portion of extended outer diameter and with at least such rim-like portion being of ferromagnetic material and a pair of slotted plates of ferromagnetic material sandwiching such apertured plate therebetween, the apertured plates and slotted plates which do not form pole structures being formed of electrically conductive, non-magnetic material and having outer diameters lying on a cylindrical surface from which said extended rim-like portions project, each ring-like permanent magnet embracing said cylindrical surface between a pair of pole structures and having opposite pole faces engaging the extended rim-like portions of such pair of pole structures. 